Portions of the following specification make reference to various patent documents and non-patent literature. The citations are listed as Patent documents and non-patent literature near the conclusion of the section entitled “Detailed Description”.
There are mainly two categories of methods of producing metal nanoparticles: chemical and physical methods. All chemical methods involve complex chemical agents for reducing the source compounds and stabilizing the colloid against coagulation. Taking gold nanoparticle as an example: one of the traditional chemical methods [Ref. 1] uses a reducing agent of sodium citrate to reduce chloroauric acid in a liquid such as water. The sodium ions also act as surfactant and prevent the gold nanoparticles from aggregation. In another traditional chemical process [Ref. 2], sodium borohydride is used as the reducing agent and tetraoctylammonium bromide is used as the stabilizing agent. Apparently a nanoparticle colloid made with these methods will contain many chemical ingredients in addition to the metal and the liquid. For many applications, these additional chemical ingredients can negatively affect the performance. For example, in biomedical and sensing applications, the stabilizing surfactants that are added during the production process can reduce the ability of gold nanoparticles to bind with those molecules that functionalize the nanoparticles for the intended application. Also, in catalyst applications, the catalytic activities of nanoparticles can be reduced by the chemical stabilizers, which reduce the effective surface area of nanoparticles exposed to the reactions.
One of the physical methods to produce metal nanoparticles is pulsed laser ablation in liquids [Ref. 3-6]. In this process, a pulsed laser beam is focused on the surface of a target that is submerged in a liquid. The ablated material re-nucleates in the liquid and form nanoparticles. This is a practically very simple and economic method. However, for the same reason of preventing nanoparticle aggregation, stabilizing chemical agents need to be added in the liquid during the ablation process [Ref. 6].
Many attempts have been tried to obtain chemically pure (i.e., free of chemical agents such as polymer, surfactant, ligand, and etc. for stabilization) metal nanoparticle colloids. For example, in a two-step femtosecond laser ablation method [Ref. 5], a Ti:sapphire laser is first used to ablate a gold target in water to produce a colloid. The colloid is then irradiated by the same laser for an elongated time up to 2 hrs. It is believed that the white light super-continuum induced by the intense ultrashort laser pulse in water can fragment large particles into nanoparticles and prevent coagulation.
For applications in photonics, another issue is to obtain tunable plasmon resonant frequency, primarily with gold nanoparticles. One suggested way is to vary the gold nanoparticle sizes, but the amount of shift of the resonant frequency is limited. An alternative way is to form alloy nanoparticles. It is expected that by adjusting the alloy composition, other physical properties such as plasmon resonance wavelength can be tuned accordingly. Again, the issue of preventing particle aggregation needs to be addressed in both chemical and physical methods of making alloy nanoparticles.
For laser-ablation based physical methods, certain production rates may be required. Laser power and pulse repetition rate are factors that limit production speed. The repetition rate can be particularly relevant because the amount of material removed by each laser shot is limited by the target material's absorption length at the laser wavelength. Standard solid state pulsed lasers such as Nd:YAG and Ti:Sapphire have very high pulse energy, ranging from milli-Joule to Joule, but a limited pulse repetition rate, ranging from 10 Hz to 1 kHz. In the two-step femtosecond laser ablation method introduced in [Ref. 8], the long irradiation time in the second step further limits the production rate.
As used herein, a stable colloid refers to a colloid having nanoparticles that do not aggregate during an extended time period after production. Such an extended time period may be at least one week, and more preferably longer than one month. By way of example, a red color of a gold colloid will be preserved for at least one month, and the colloid may be characterized with optical absorption spectroscopy measurements.
As used herein, a chemically pure colloid refers to a colloid that contains only a liquid and nanoparticles. Such a chemically pure colloid does not require an additional chemical agent to prevent aggregation among nanoparticles, and does not require such a chemical to stabilize the colloid against coagulation. By way of example, a chemically pure gold-water colloid contains only water and gold nanoparticles, and is substantially free of stabilizing agents, such as a polymer, surfactant, ligand, or similar agents.